This invention relates to color picture tubes having tension masks, and particularly to a tube with a tension mask that is made of a low expansion material.
A color picture tube includes an electron gun for generating and directing three electron beams to a screen of the tube. The screen is located on the inner surface of a faceplate of the tube and is made up of an array of elements of three different colors emitting phosphors. A color selection electrode or shadow mask is interposed between the gun and the screen to permit each electron beam to strike only the phosphor elements associated with that beam. A tension mask is a thin sheet of metal, such as steel, that is contoured or stretched under tension to somewhat parallel the inner surface of the tube faceplate.
A problem that must be solved in tubes utilizing tension masks is that of the loss of tension during operation, caused by thermal inputs, such as vertical blister bars. Vertical blister bars are bright areas on an otherwise dark screen that are about 3 cm wide and about 15 to 25 cm long. In the past, this problem was solved by placing the vertical mask strands of a steel mask under tensions ranging as high as 45 ksi. These high tensions produce enough strain in steel masks to overcome the thermal expansion caused by a blister bar, and to retain adequate tension under most operating conditions. However, the higher electron beam power available in modern television receivers has made the available tolerance in tension masks to thermal expansion unacceptable in some operating conditions. The high stress in a steel tension mask requires a massive mask support frame to provide the necessary tension forces to the mask. Such masks are high in both cost and weight. The high stresses in the mask and frame also require special mask and frame materials that have low thermal creep properties, thereby further increasing their costs. Furthermore, steel tension masks also require some detensioning means during high temperature processing.
The present invention recognizes that a lighter frame can be used in a tension mask tube, if the required tension on a mask is reduced. One way to reduce the required mask tension is to make the mask from a material, such as Invar, having a low coefficient of thermal expansion.
The present invention provides an improvement in a color picture tube having a tension mask attached to a support frame. The improvement comprises the mask being made from a material having a significantly lower coefficient of thermal expansion than the coefficient of thermal expansion of the material of the frame. The frame tensions the mask to have a fundamental resonant frequency of 90 Hzxc2x120 Hz.